1. Field of the Invention
The present invention is directed to a device and a method for conducting a function test on control members of a turbomachine and particularly for performing a partial stroke test on valves.
2. Description of the Prior Art
US 2006/67810 A and U.S. Pat. No. 4,512,185 disclose test systems for testing one of a plurality of control valves through which a work medium is supplied to a turbine in its entirety. When the control valve under test is closed, the corresponding proportion of work medium is supplied by the rest of the control valves so that turbine operation is not affected.
In many technical systems, certain control members or process control elements such as, e.g., valves, remain in an unchanged position for long operating periods. This is the case particularly for process control elements which are to be used only in emergencies or in failure situations. Because the process control elements are rarely actuated, partial or complete blockage may occur, for example, due to deposits, wear, or thermal deformation. When this process control element is a valve which is only opened in case of emergency, this valve which is defective due to blockage will remain unnoticed until actuated in an emergency.
An example of a protection system of this kind is a surge limit control valve in turbocompressors. The surge limit control valve protects against surge, as it is called. Surge occurs when the operating point of the turbocompressor enters the unstable region of the operating map due to reduced flow or because of a rise in the end pressure. Surge brings about a cyclical flow and reverse flow of the compressed gas. This results in strong vibrations, pressure shocks, and rapid rises in temperature in the turbocompressor. Possible consequences include damage to bearings, rotors or blades and related operating outages. The surge limit control valve must be checked regularly for proper functioning because it is seldom actuated as it is only used for emergencies.
In the prior art, this problem in valves is countered by the so-called partial stroke test. In this partial stroke test, the valve is actuated at regular timed intervals in such a way that it travels over a small portion of its lifting movement. The lifting movement of the valve is monitored and its functionality is tested in this way.
The disadvantage in the partial stroke test known from the prior art consists in that the actuation of the valve influences the operation of the entire process downstream of the turbomachine. For this reason, only a short stroke is traveled, or the testing time is reduced to a minimum in order to keep the influence on operation as slight as possible. Surge limit control valves are usually designed for approximately 200% of the rated compressor capacity. If the surge limit control valve opens by only 5%, then 10% less of the flow is delivered to the process. This represents an unacceptable operating failure for many processes. On the other hand, in many applications a test of less than 5% of the valve lift does not offer sufficient certainty that the valve will also work safely along a greater stroke region.
The partial stroke test does not rule out the possibility that a valve which has traveled the partial stroke properly without a problem may nevertheless become blocked when departing from this partial stroke. In tests of minimum duration and, therefore, abrupt adjustment of the valve, incipient blockage which expresses itself, for example, in an inhomogeneous movement of the valve in a slow adjustment of the valve cannot be determined.